EP0086014A1 - Method for the production and stabilisation of a reversible two-way memory effect in a Cu-Al-Ni or Cu-Al alloy - Google Patents

Abstract

Process for producing a reversible two-way effect in Cu / Al / Ni and Cu / Al memory alloys by heat treatment with simultaneous application of an external stress (5) after the solution annealing, quenching (1; 2) and deformation (3) were carried out by the conventional steps Martensite formation. Additional improvement through optional martensite stabilization (9) and two-way effect zero point stabilization (11) in the form of further heat treatments.

Description

The invention is based on a method for generating and stabilizing a two-way memory effect according to the preamble of claim 1.

In the case of memory alloys, a so-called two-way effect can generally be distinguished from a one-way effect. While the latter is generally more pronounced and well-known (Ni / Ti alloys, β-brasses) and has also led to numerous applications, the two-way effect is more problematic and difficult to master. However, there is a general need in technology for components which have a sufficiently large two-way effect in terms of quantity in order to open up another interesting area of application. Most of the time, however, the point of the martensitic transformation of the classic two-way effect alloys lies in an unfavorable temperature range. However, there are a number of memory alloys, above all the classic Cu / Al / Ni and Cu / Al alloys belonging to the β-brass type, whose transformation point is favorable, which may have a clear one-way but hardly a well-known two-way effect.

• R. Haynes, Some Observations on Isothermal Transformations of Eutectoid Aluminium Bronzes Below Their M_S Temperatures, Journal of the Institute of Metals 1954-55, Vol. 83, Seiten 357-358; W.A. Rachinger, A "super-elastic" single Crystal calibration bar, British Journal of Applied Physics, Vo. 9, Juni 1958, Seiten 250-252; R.P. Jewett, D.J. Mack._Further Investigation of Copper-Aluminium Alloys in the Temperature Range below the β
  
  α+γ₂ Eutectoid, Journal of the Institute of Metals 1963-64, Vol. 92, Seiten 59-61; K.Otsuka and K. Shimizu, Memory Effect and Thermoelastic Martensite Transformation in Cu-Al-Ni Alloy, Scripta Metallurgia, Vol. 4, 1970 Pergamon Press Inc., Seiten 469-472; Kazuhiro Otsuka, Origin of Memory Effect in Cu-Al-Ni Alloy, Japanese Journal of Applied Physics, Vol. 10, No. 5, May 1971, Seiten 571-579.

The following documents can be cited as state of the art:

• R. Haynes, Some Observations on Isothermal Transformations of Eutectoid Aluminum Bronzes Below Their M _S Temperatures, Journal of the Institute of Metals 1954-55, Vol. 83, pages 357-358; WA Rachinger, A "super-elastic" single crystal calibration bar, British Journal of Applied Physics, Vo. June 9, 1958, pages 250-252; RP Jewett, DJ Mack. _For ther Investigation of Copper-Aluminum Alloys in the Temperature Range below the β
  
  α + γ ₂ Eutectoid, Journal of the Institute of Metals 1963-64, Vol. 92, pages 59-61; K.Otsuka and K. Shimizu, Memory Effect and Thermoelastic Martensite Transformation in Cu-Al-Ni Alloy, Scripta Metallurgia, Vol. 4, 1970 Pergamon Press Inc., pages 469-472; Kazuhiro Otsuka, Origin of Memory Effect in Cu-Al-Ni Alloy, Japanese Journal of Applied Physics, Vol. 10, No. 5, May 1971, pages 571-579.

There is therefore a need for components made of memory alloys of the β-brass type, which have a well-known two-way effect at a transformation temperature which is favorable for certain applications.

The invention is based on the object of specifying a method for Cu / Al / Ni and Cu / Al alloys in order to induce a considerable reversible two-way memory effect in these substances and, if appropriate, to stabilize this effect in such a way that from the said Alloys components that can be used in practice can be produced.

This object is achieved by the features specified in the characterizing part of claim 1 and additionally in a preferred manner by the features specified in the characterizing part of claim 2.

The invention is described on the basis of the following exemplary embodiments explained by figures.

• Fig. 1 ein Fliessdiagramm des Verfahrens,
• Fig. 2 ein Zeit/Temperatur-Diagramm des Ablaufs der einzelnen Verfahrensschritte,
• Fig. 3 ein Diagramm des Zweiweg-Gedächtniseffekts am Beispiel eines Biegestabes,
• Fig. 4 das Schema einer Versuchseinrichtung für Biegestäbe.

It shows:

• 1 is a flow diagram of the method,
• 2 shows a time / temperature diagram of the sequence of the individual process steps,
• 3 shows a diagram of the two-way memory effect using the example of a bending rod,
• Fig. 4 shows the scheme of a test facility for bending rods.

1 shows the flow diagram of the method in individual steps in block form. The decisive thermomechanical or heat treatments are highlighted by a frame. The diagram does not need any further explanation. Process example in parentheses

FIG. 2 shows a time / temperature diagram of the sequence of the individual method steps corresponding to the flow diagram of FIG. 1. 1 represents the usual solution annealing mostly at approx. 850 ° C, ie the transformation of the alloy into the structure of the / 3 -mixing crystal, 2 the subsequent water quenching to carry the metastable state to room temperature. 3 is to achieve a memory effect and for shaping of the component necessary critical deformation step, which can be carried out at room temperature or in principle at any temperature below 300 ° C. After this deformation, the workpiece does not necessarily have to be relieved. In the event of deformation above room temperature, it can be cooled. 5 is the step of shape stabilization, which is carried out at 150 to 425 ° C. (in the present case 300 ° C.) under tension, ie with simultaneous application a load has to take place. Subsequently, cooling can be carried out slowly according to 6. From step 3, the state according to step 5 can optionally also be reached via step 4 (maintaining the temperature). 7 and 8 illustrate an optional one-way effect treatment followed by slow cooling. However, these steps can also be omitted. The additional advantageous step of martensite stabilization with subsequent slow cooling is shown in sections 9 and 10. Finally, there is the optional, but advantageous two-way effect zero stabilization according to section 11 and the slow cooling 12.

Fig. 3 shows a diagram of the two-way effect using the example of a bending rod. The deflection (deflection) f in mm is plotted as a function of the temperature T in ° C. If you go through a temperature cycle between about room temperature and 200 ^o C, the reversible deflection that follows the hysteresis curves is achieved. Curve 13 corresponds to the effect without, curve 14 to that with martensite stabilization. The quantitative improvement due to martensite stabilization is clearly evident. The maximum achievable deflection difference of approx. 5 mm between the high and low temperature phase corresponds in the present case to an elongation S of approx. 1.3%. 4 schematically shows a test device for bending rods. 15 is the bending rod made of memory alloy. 15 represents a clamping device for the bending rod, that is to say the so-called fixed point. 17 is a cord which is guided over a cable pulley 18 and by means of a counterweight 19 is slightly tense. The cord 17 is attached to the movable end of the bending bar 15. The counterweight 19 is dimensioned such that it is only with certainty that the frictional forces when moving process overcomes. The arrow represents the direction of movement of the deflection of the bending rod 15, which, with full deflection (deflection), reaches the position according to FIG. 21 with its axis. The movements can be read or registered on a sensor (not shown) mounted on the roller 18.

Embodiment I:

A bending rod of the following composition was used to generate and measure a two-way memory effect:

The rod had a square cross section of 2.5 x 2.5 mm and a length of 35 mm. It was treated similarly to the procedure shown in FIGS. 1 and 2. First, the rod was pre-shaped at a temperature of 900 ° C in such a way that its longitudinal axis described a circular arc with a radius of -37 mm (negative sign, since curvature in the opposite direction with respect to the later definitive shape). The pre-bent rod was then subjected to solution annealing at a temperature of 950 ° C. for 15 minutes and subsequent quenching in cold water (similar to 1 and 2 in FIG. 2). The rod was then bent in the opposite direction at room temperature so that its longitudinal axis described a radius of + 35 mm. This corresponded to an outermost fiber elongation of 6.88% (similar to 3 in Fig. 2). The rod was then subjected to a shape stabilization under tension by heating it under load for 30 minutes to a temperature of 300 ° C. in such a way that the clamping ensured that the radius of its longitudinal axis was kept at + 35 mm (similar to 5 in Fig. 2). After slow cooling, the bending bar was freed from the previously applied load. Finally, the rod was subjected to martensite stabilization at 300 ° C for 30 minutes (similar to 9 in Fig. 2). After slow cooling, the rod was tested in the arrangement shown in FIG. 4. The behavior corresponded approximately to curve 14 in FIG. 3 on average. The difference in deflection (deflection) on several test rods, which was run through in a temperature cycle, fluctuated between 4.4 mm and 5.9 mm, which corresponds to an elongation of 1.15% to 1 , 53% corresponded. The corresponding values were 4.94 mm and 1.28% on average. The lower transition temperature was on average approx. 160 ° C, the upper approx. 177 ° C, the hysteresis width thus approx. 17 ^o C.

Example II:

A test bar of the same dimensions and the same composition as in Example I was subjected to solution annealing at 850 ^° C. for 10 minutes in the flat (stretched) state and then quenched in cold water. The rod was then bent to a radius of 22 mm at room temperature, which corresponded to an outermost fiber elongation of 5.4%. The bent rod was then held at a temperature of 300 ° C for 30 minutes under load. The load was removed while still warm and the relieved rod was slowly cooled to room temperature. The test was carried out in the same manner as in Example I. With this material without "martensite stabilization", greater scatter and less two-way effects were found than with the stabilized one according to Example I. The difference in deflection on several test specimens, which was carried out in one temperature cycle, fluctuated between 2.6 mm and 5.8 mm, which corresponds to an elongation of 0 , 70% to 1.32% corresponded. The corresponding values averaged 4.20 mm and 1.08%. The lower transformation temperature was on average approx. 107 ° C, the upper approx. 150 ° C, the hysteresis width thus approx. 43 ° C.

Working example III:

A torsion bar of the same composition as in Example I was subjected to a corresponding treatment and test. The rod had a round cross-section, with a diameter of 3 mm and a measuring length of 24 mm. It was first solution annealed at a temperature of 850 ° C for 10 minutes and then quenched in cold water. The rod was heated to 100 ° C and twisted (twisted) at this temperature by a total angle θ of 80 ^o with respect to the circular cross sections at the ends of the measuring length. The twist angle α of the outermost fiber of the helix with the cylinder generating line (pitch angle) was approximately 5 °, which corresponded to an elongation of 6% in the main stress directions (tension and compression). The rod was held in this tensioned position and heated to a temperature of 300 ° C. This state was maintained for 20 minutes. Then the rod was relieved and slowly cooled to room temperature. The two-way effect on torsion was then determined by going through a temperature cycle of 0 to 250 ^° C. The achievable reversible angle difference Δθ of the circular cross sections at both ends of the measuring length was 9 °, which corresponded to a twist angle cC (pitch angle) of 34 '. The equivalent of the strains in the two main stress directions (tension and compression) was thus about 0.7%.

Working example IV:

A tensile bar with the same composition and the same dimensions as the torsion bar according to Example III was subjected to a corresponding treatment and test. It was first subjected to solution annealing at a temperature of 850 ° C. for 15 minutes and then quenched in cold water. Then the rod was subjected to tensile stress in its longitudinal axis at room temperature and stretched (stretched) by an amount of 4%. While maintaining the applied load (tensile stress), the rod was heated to 300 ^° C. and held in this state for 20 minutes. The load was then removed and the rod slowly cooled to room temperature. The two-way effect measured on various test specimens in the temperature interval from 0 to 200 ^° C. was 0.2 to 0.5%. It could be significantly improved by the martensite stabilization treatment according to 9 in FIG. 2 and kept within narrow limits.

The invention is by no means exhausted in the above-mentioned exemplary embodiments. In principle, any memory alloy of the β-brass type, which shows a well-known one-way effect in the natural state (i.e. after conventional treatment), but shows a negligible two-way effect, can be brought into a state according to the new method, where it has a clear two-way effect suitable for practical use having. These primarily include the alloys of the types Cu / Al / Ni and Cu / Al.

Depending on the alloy and size of the workpiece, solution annealing can be carried out at different temperatures (usually between 850 and 950 ° C). The decisive process step corresponding to the shape stabilization can in the temperature range of 150 to 425 ° C for 0.5 to 180 min, the shorter times apply to the higher temperatures. The load to be applied at the same time must be dimensioned such that a tension (tension, pressure or thrust) is generated which corresponds to an elongation of at least 1%. In the case of thrust, this means that the displacement angle α (pitch angle for torsion) must be at least 50 '. The martensite stabilization can be carried out in the temperature range between 200 and 400 ° C for 1 min to 4 h. The zero point stabilization of the two-way effect is advantageously carried out at a temperature which corresponds to the later maximum operating temperature (in our case approx. 200 ^° C.). The duration of the heating should be at least 1 min.

The present new method has shown the way for the first time how a two-way effect that can be used in practice can be achieved with memory alloys of the β-brass type, which normally only show a pronounced one-way effect.

Claims (3)

1. A method for producing and stabilizing a reversible two-way memory effect in a Cu / Al / Ni or a Cu / Al alloy, the alloy produced by melt metallurgy or powder metallurgy being first subjected to solution annealing in the temperature region of the β mixed crystal with subsequent quenching in water , whereupon the alloy is subjected to deformation, characterized in that the alloy after the deformation to stabilize the shape of a heat treatment in the temperature range from 150 to 425 ^° C. for 0.5 to 180 min with simultaneous application of a load to generate a tensile, compressive or subjected to shear stress corresponding to an elongation of at least 1%, slowly cooled to room temperature and relieved and subjected to one or more further process steps. 2. The method according to claim 1, characterized in that as a further process step after slow cooling and relief, the alloy for martensite stabilization is subjected to a heat treatment in the temperature range from 200 to 400 ⁰ C for 1 min to 4 h and a slow cooling to room temperature. 3. The method according to claim 2, characterized in that as an additional step after the heat treatment for martensite stabilization, the alloy to zero Point stabilization of the two-way memory effect is subjected to heat treatment at a temperature corresponding to the maximum operating temperature of the component for at least 1 min and slowly cooled to room temperature.

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